This invention relates to an improved two-part adhesive composition useful as a structural adhesive in forming transparent or clear bond lines. More particularly, this invention involves an improved adhesive composition wherein one-part comprises a polymerizable acrylic monomer and an organoboron amine complex and the second-part contains an organic acid destabilizer and optional polymerizable acrylic monomer wherein the improvement comprises using about 45% or more by weight of selected phenyl containing acrylic compounds as the acrylic monomer.
Adhesive compositions including acrylic adhesives such as solutions of elastomeric polymers in soluble polymerizable acrylic or methacrylic monomers are well known in the art. These compositions which are especially known for their toughening properties generally include a redox system which comprises a catalyst or initiator, an accelerator and an activator to initiate cure, as well as other additives such as adhesion promoters, chelators, cross-linking agents, thickeners and plasticizers. Two-part acrylic adhesives where an activator is added as a separate second component are widely used and are known for curing speed as well as toughness. The catalyst or initiator typically used in these adhesives are free-radical initiators of the organic peroxy or hydroperoxy type, perester or peracid type.
Many known adhesive systems, such as the epoxies, require a thermal cure to obtain desirable properties, while others that do not, usually require prolonged cure times. More recently adhesive compositions with organoboron amine complex initiators have been developed which cure at room temperature and reach high tensile strength in a short period of time. These new adhesive compositions which have shown exceptional stability on ageing while exhibiting both high tensile strength and high peel strength are disclosed in U.S. Pat. Nos. 5,106,928 and 5,143,884 issued to M. Skoultchi, et al. on Apr. 21, 1992 and Sep. 1, 1992 respectively. This adhesive is useful in structural systems particularly those requiring fast curing at room temperature. However, when such adhesives cure they form an opaque bond-line making them less desirable for use with clear substrates such as glass and clear plastics.
Accordingly, there is need for an adhesive composition which provides transparent or clear bond lines upon curing making it particularly useful in structural systems employing glass, plastic or other clear substrates.